2.4.1 Thermodynamic assessment Energy and exergy analyses
The 1st Law of Thermodynamics indicates that energy can neither be created nor destroyed, and it allows therefore for tracking energy flows in the forms of power, heat or matter. However, it does not account for the thermodynamic quality of different forms of energy, implying that all forms are taken as equivalent. It yields only limited information on the maximum system performance that could be reached in theory. Processes such as isenthalpic expansion with throttling valves or heat transfer in adiabatic heat exchangers seem to not present any potential for improvement.
In contrast, the 2nd Law of Thermodynamics addresses these gaps, as it asserts that in real en- ergy transformations, a particular system spontaneously evolves in a given direction, towards thermodynamic equilibrium. Real processes are therefore irreversible, and the measure of these internal inefficiencies can be performed by using the property of entropy. Entropy is generated as a system undergoes transformations until reaching equilibrium, implying that the equilibrium state is attained as the entropy reaches a maximum. It indicates that the heat input into a given system cannot fully be converted into useful work, and that the possible uses of energy from sources such as low-temperature heat are limited, unlike electricity.
The combination of these two laws leads to the concept of exergy, and the following definition has been proposed by Szargut et al. [44]: Exergy is the amount of work obtainable when some matter is brought to a state of thermodynamic equilibrium with the common components of the natural surroundings by means of reversible processes, involving interaction only with the abovementioned components of nature. As real processes are, in essence, irreversible, exergy is destroyed as the system evolves towards equilibrium, and the quantity of destroyed exergy accounts for the system inefficiencies, while giving a clear picture of the resources that are consumed and degraded.
The concept of exergy goes back to the contributions of Clausius, Thomson, Gibbs, Gouy and Stodola in the 19th century, where the term of available energy was first used. The modern development of this type of analysis was initiated by Bosnjakovic in Europe around the 1940s and the term of exergy was coined by Rant in the 1950s to denote the capability for work extraction. The exergy analysis method is closely linked to the concept of exergy-based performance criteria, which would provide a more consistent measure of the resource use, and which would thus be more appropriate for evaluating process performances. This concept has strongly evolved, with the contributions of Grassmann [45] and Nesselmann [46] in the 1950s, of Baehr [47, 48] in the 1970s, of Kostenko [49], Tsatsaronis [50] and Szargut et al. [44] in the 1980s, to the most recent ones of Lazzaretto and Tsatsaronis [51, 52] in the last decade.
systems was published from the 1970s, along with several reference textbooks on the matter (see e.g. Szargut et al. [44], Moran [53] and Kotas [54]). The application of exergy analyses to petroleum systems was more recent, starting from the 1990s, and primarily focused on crude oil distillation processes, with the studies of Rivero and Anaya [55], Cornelissen [56], Demirel [57] and Al-Muslim and Dincer [58].
The first work on the thermodynamic performance of oil and gas offshore processing was the exergy analysis of de Oliveira Jr. and Van Hombeeck [59], which deals with the case of a Brazilian facility, where petroleum is extracted at a low temperature and exported ashore, along with gas. The separation sub-system was the most inefficient process, and the crude oil heating was the most exergy-destroying one.
Pinch and total site analyses
The pinch analysis method builds as well on the 1st and 2nd Laws of Thermodynamics and takes its roots in the work of Linnhoff [60], who pointed out some of the limitations of the exergy concept when applied to the case of heat exchanger networks. The main focus is on targeting the heat integration potential in the system under study. These methods are suitable for designing new processes (grassroot) [61] or for improving existing ones (retrofit) [62]. The pinch analysis method has been extended from the analysis of an individual process [63] to the assessment of a total site [64–67]. The energy saving opportunities are assessed at the site-scale, rather than at the level of a single process. The opportunities for inter-process integration, through a common utility system, can be identified [68].
Process integration tools for improving the energy efficiency of industrial sites have been applied to different sectors, ranging from the pulp and paper sector to the petrochemical and fine chemical industry. Parallels have been developed for other types of networks, such as distillation systems, water distribution [69, 70] and hydrogen refineries [71, 72], with the con- stant aim of using more efficiently raw materials or energy, while reducing the environmental impact and emissions of the diverse process operations.
The application of these methods to petroleum and large-scale plants has grown recently. Feng et al. [73] investigated possible retrofit schemes for heat exchanger networks of petro- chemical plants and pinpointed the importance of choosing the relevant boundaries (process or site). Matsuda et al. [74] applied the total site approach to a large-scale plant, revealing the significant energy saving potential of the facility, despite the high efficiency of each individual process. Chew et al. [75] discussed the implementation issues of this method when applied in practice. However, there is no study dealing with the energy integration of oil and gas facilities.
As emphasised in Bejan et al. [76], the use of pinch analysis may be limited for detecting improvement opportunities in systems in which chemical reactions are involved. Pinch and exergy assessment methods should therefore be considered as complementary, since they both provide deeper insights into the performance of energy systems.
2.4.2 Economic analysis
An economic analysis may be defined as:
The systematic approach in which economists and other professionals will estimate the eco- nomic environment and its strengths and weaknesses.
Evaluating the economics of chemical processing plants has been the subject of a wide range of scientific literature in the last decades, and the purchased costs of the equipment items can be deduced from estimating charts, such as the ones presented in Timmerhaus et al. [77] or in Ulrich [78], or capacity-based correlations, such as the ones presented in Turton et al. [79].
In general, these works pinpoint that the evaluation of the economics of a chemical process, new or existing, builds on the assessment of the capital and operating costs associated with the construction, operation and decommissioning.
The economic evaluation of oil and gas platforms is trickier and calls for flexibility when investing in a new facility [80], as there are significant uncertainties associated with, for instance [81]:
(1) the oil and gas prices [82], and the financial market volatility [83];
Volatility in the oil prices has a clear negative impact on investment measures, and these prices may vary significantly over the lifespan of the petroleum field, because of politic (e.g. potential market disruption because of wars and geopolitical tensions) and economic factors (e.g. economic crisis). This uncertainty should therefore be considered to assess reasonably the project profitability.
(2) the investment (CAPEX) and operating (OPEX) costs, as well as the inflation rate;
(3) the production profiles and reserves of petroleum [84];
The production profiles strongly depend on the reserves and reservoir conditions. Not all the oil present in the reservoir is recoverable technically, because of the reservoir geological conditions and limitations in the extraction technologies.
(4) the number of wells and the associated capital costs and production profiles [85]; Similarly, only a fraction of the oil that can be technically recovered is actually econom- ically interesting to extract, because of the additional investment costs that would be induced if new wells were drilled.
(5) the start and development of the production.
Finally, there are uncertainties associated with the production volumes during the build-up, plateau, decline and abandonment phases: each life stage presents different uncertainties, and the durations of each phase vary from one field to another.
2.4.3 Environmental assessment
Senécal et al. [86] proposed to define environmental impact assessment methods as:
The process of identifying, predicting, evaluating and mitigating the biophysical, social and other relevant effects of development proposals prior to major decisions being taken and com- mitments made.
Such methods aim at addressing environmental considerations in decision-making processes, in order to evaluate the sustainability of a given project or plan. An example of such a method is the life-cycle analysis, mainly developed from the mid-1980s, which consists of evaluating the environmental impacts during all the life stages of a product or process.
The term life cycle refers to the main lifespan stages, from the processing of raw materials to the waste management and disposal. The exact procedure is at present standardised by [87] and is well-described in e.g. Rebitzer et al. [88, 89]. It consists of an inventory of the relevant inputs and outputs of material and energy streams, the environmental impacts associated with each flow, and an interpretation of the results. In general, the latter include the consumption of material and energy resources, and an assessment of the environmental impacts.
However, a large range of methodological issues has raised, as different practitioners use different assumptions for the same type of problem (e.g. system boundaries and information sources), leading to inconsistent results [90]. Finnveden et al. [91] discussed the most recent developments of this technique and pinpointed the need for an extensive amount of data and the uncertainties caused by the methodological choices.
McCann and Magee [92] performed a comparison of the life cycle of seven different crude oils, which have different origins and chemical properties, implying that the processing schemes are different, especially when it comes to the refinery treatment.
The U.S. Environmental Protection Agency [93] conducted an extensive study of the envi- ronmental impacts of oil and gas production, based on a regional analysis of some of the U.S. states. They reported that the main environmental impacts were associated with the air emissions, produced water effluents, and drilling waste flows, because of the emissions of nitrogen, sulphur oxides, methane and carbon dioxide from the combustion sources and separation vessels.
Venkatesh et al. [94] analysed the uncertainties related with the estimations of greenhouse gas emissions during the life cycle of petroleum-based products and showed that the crude
extraction and transport sector represented about 10 % of the total CO2,eqemissions for such
fuels, and that the coefficient of variation was about 43 %.
Burnham et al. [95] compared the sources and extents of greenhouse gas emissions for hydro-
carbon fuels, applying a life-cycle analysis. In the case of natural gas, most CH4emissions were
2.4.4 Hybrid methods Exergy and economics
Several methods have been developed these last years as combinations of thermodynamic, economic and environmental evaluations. The term thermoeconomic stands for the combina- tion of thermodynamic and economic variables, while the term exergoeconomics was coined by Tsatsaronis [50] to denote the particular combination of an exergetic and an economic assessment. The main idea lies on valuing energy and material flows based on their associated exergy to design a system. The exergetic cost [96] represents the quantity of exergy that is needed to produce a given flow or product.
These methods have been extended to analyse and optimise existing energy systems [97], and to detect operation anomalies [98, 99]. The application of exergoeconomic and thermoeco- nomic tools to oil and gas systems is limited. Rivero et al. [100] evaluated a crude oil combined distillation unit, while Nakashima et al. [101] performed an exergoeconomic evaluation of the Marlim platform to compare two production techniques. Silva et al. [102] derived the production costs of petroleum-derived fuels.
Exergy and environment
The exergy concept may also be used to assess the ecological cost of using raw materials or the impact of waste emissions. Rosen and Dincer [103, 104] suggested that a large number of environmental issues can be correlated to the conversion of energy sources. Dewulf et al. [105] suggested to (i) evaluate environmental impacts by calculating the quantity of exergy required to abate the corresponding emissions in waste treatment plants, or (ii) by evaluating the losses of exergy due to health effects [106].
Szargut and Morris [107] introduced the concept of cumulative exergy consumption, which is defined as the consumption of energy carriers in all the steps of the production processes from natural resources to final products. More specifically, Szargut et al. [108] defined the term thermo-ecological cost as the cumulative consumption of non-renewable exergy.
Gong and Wall [109] stressed that the concept of exergy can be embedded in the life cycle analysis method under the name of exergetic life cycle analysis, in order to assess the exergy inputs and outputs during the construction, operation and clean-up phases. Cornelissen and Hirs [110] noticed that this method could be used to determine the consumption and the depletion of natural resources. De Meester et al. [111] suggested calculation improvements, as the current datasets given in the literature might have resulted in significant uncertainties.
Meyer et al. [112] developed the exergoenvironmental analysis, which combines the outputs from the life cycle and exergetic assessments, but focuses on the environmental impact forma- tion at the level of the plant components. Dewulf et al. [113] reviewed the different applications of exergy and argued that using this concept in efficiency accounting is appropriate.